Lower tube cracking

[ryannmphs]

OK, over the weekend I turned 3 pens plus I had one all ready to be assembled. So last night I'm assembling these 4 pens (padauk,blk walnut, maple, and and unknown wood). 2 of the 4 pens cracked while inserting the twist mechanisim. These were all slimlines. I had visually checked the inside of the tube before starting the assembly process and did not see anything.

I had turned the pens and finished with BLO & CA. This was really frustrating because the tubes did not crack until after the twist mechanisim was seated. I actually felt one crack and did not notice the other until this morning.

What can I do to help reduce the chances of more pens cracking on me?

Ryan

 

[Fred in NC]

Several possible causes:

The brass tube expands a tiny little bit when the metal part is inserted. If the tube fits too tight in the wood, the wood can crack as the tube expands. This happens more with some woods than others. Make sure the tube is not too tight before it is glued in.

Even a little glue inside the end of the tube can make it expand more than necessary. I don't trust my eyes. I clean the end of the tube with a size 'D' drill bit. I put a wood handle on the bit. It is about .246", which is the size of the mandrel.

The metal parts have to be straight when they are inserted. I line them up by hand, and keep an eye while pressing them in.

One more... the tubes are too small ! this can happen if you mix the parts from different manufacturers.

Hope this helps.

 

[dw]

Nothing says you can't take a jeweler's file and reduce the diameter of the fitting a little bit. Most every fitting (but, admittedly, not all) could stand to be a tidge smaller in diameter and no harm would result. And where the tightness of the fitting is critical...or where you've perhaps (oh no!) reduced the diameter a little more than necessary, a tiny bit of five minute epoxy will work wonders.

When I go to press in a fitting, I examine the wood left around the tube (how thick is it?) and I think about what kind of wood it is (how hard/brittle is it?). Then I try gently pushing the fitting into the tube...with a press. If it seems too tight...even if I'm imagining it's too tight...I'll reduce the diameter of the fitting.

Better safe than sorry.

 

[dougle40]

Another problem could be the combination of either a small tube or a slightly oversized fitting along with the CA finish . When CA is applied to wood it penatrates the wood and almost turns it into something resembling a hard plastic that won't expand like a natural wood will . This would come into effect even more since the pens were slim line pens with very little wood left after turning . That fact is proven when you use CA to stabilize a piece of wood , you can still turn it with no problems . It becomes extremely hard .

 

[wayneis]

If you use CA to glue the tubes into the blanks it's very hard to see if you happened to get a little inside, boom blow out. Someone pointed out a while ago about the brass gun cleaning brushes and I bought one of each size. Now before I turn a blank I chuck the brush into my cordless drill I run the brush through the tube from both ends. It does two things, first it eliminates any glue which will cause off center turning and to tight of fittings and it takes off any burr's from squaring the blank.

Wayne

 

[Fred in NC]

I am not using CA to glue the tubes anymore. Epoxy gives me kind of a cushion between the tube and the blank. I have not had any more cracked barrels since I started using epoxy, and cleaning the inside of the barrel (in my case, with a D drill bit, other tools will work too).

 

[Scott]

OK Ryan, lets summarize this.

Most likely your lower pen barrel is cracking when you press in the mechanism because it is causing the brass tube to expand slightly, which cracks the thin wood. Some of the causes of this are:

* Residual glue in side the brass tube.
* A too small brass tube.
* A burr at the opening of the brass tube.
* Not enough room between the brass tube and the wood.
* A hard, unyeilding glue which communicates the stress to the wood, which is weaker.
* The wood at that point is too thin or brittle, and cracks with very little stress.

There are a lot of ways to alleviate most of these problems. Consider drilling with a 9/32 bit instead of a 7mm, which gives a touch more room inside the blank. Consider using polyurethane glue, such as Gorilla glue, or maybe epoxy, which has more cushion to it. Remove the burr at the end of the brass tube with either a small knife or one of Bill Baumbeck's de-burring tools. Consider getting a reamer to open up the brass tube, or just use a small file like most of us. Clean out all glue that may be inside the brass tube with whatever is handy for you. If you are making a thin slimline, avoid using materials that are prone to cracking.

I don't know which, if any, of these apply to your pens. But among these you will find an answer that will work for you. Just take some time and look at your pen barrels, and see what is happening when you press in the parts. Wood that thin is not terribly strong, and it doesn't take much to crack it!

Good Luck!

Scott.

 

[dougle40]

If your worried about getting glue in the tube there's an easy way to prevent this from happening . Before you insert the tube , stick the ends in a piece of potato sliced about 1/4" thick , then insert the tube and no glue will get into it . When the glue is dry simply knock out the potato plugs .

 

[low_48]

Epoxy is a good gap filler, but absolutely not a cushion. It is an extremely brittle adhesive. No give whatsoever.

Rich

 

[ryannmphs]

Thanks for helpful hints, I will use them on my next batch. I actually used Gorilla Glue this time, instead of the CA for glueing the tubes in the blank.

I thought the barrel trimmer cleaned the inside of the tube? I'll have to look the next time I go to the HD or Lowes for the size 'D' bit, and I will start plugging the tube while glueing in the blank

Again, thanks for the tips and suggestions.

Ryan